"Old Man's War" comes from a long tradition of military novels: "Starship Troopers" and "The Forever War" (Haldeman), just to name two. Scalzi produced a military novel but in modern attire: tale of alien conflict and dressed it up intelligently with such themes as individual identity, what makes us human, the significance of mortality, and the ethics of life extension.

It's all in there. If you want to recapture the old Heinleinian spirit, this is the book to go to.